Optical transmission systems, which are based on wavelength division multiplexing (WDM), achieve high information capacities by aggregating many optical channels, each with a unique carrier frequency, onto a signal strand of optical fiber. Tunable filters play a critical role in WDM communication systems. A tunable filter, which can redirect and route light signals of certain wavelengths (channels), is used:                in conjunction with a tunable laser to create a tunable transmitter,        midway in the fiber in wavelength for add and drop multiplexing applications,        at the receiving end in conjunction with a broad band detector for a tunable receiver.        
In applications utilizing add and drop multiplexing, the tunable filter is often termed a three or more port device, with input, express and add (drop) ports. In these applications, the network traffic enters an optical device at the input, with most of the channels leaving at the express port. The dropped channels are redirected to the drop port, while the added channels are input from the add port. The filter is characterized by the following key parameters: bandwidth, attenuation (rejection) of out of band signal, tuning range, required power, hitless tuning, and free spectral range.
Known techniques of the kind specified provide for the following two categories of implementing three- or four-port tunable filters:
Providing spatial distribution of the different channels, and switching of the channel(s) to be dropped. Here, tunability is achieved by applying spatially distinct switches, which switch different channels to the drop port [U.S. Pat. No. 5,771,112].
Changing the frequency of operation by physical changes in the optical filter medium. This is the so-called scanning unable filter, since it scans over frequencies [U.S. Pat. No. 6,091,870].
For most applications, the scanning filter presents a cost-effective solution and is the preferred method of realization of the tunable filter for add/drop multiplexing. However, most of the known implementations of scanning filters fall short of fulfilling the required specifications, because of inherent material physical limitations.